Hydrogen fuel cells have emerged as promising devices for clean and efficient generation of power for global energy needs. Although hydrogen fuel cells have a low impact on the environment, current hydrogen production technologies rely on high-temperature steam reforming of non-renewable hydrocarbon feedstocks. Greater environmental benefits of generating power from hydrogen fuel cells could be achieved if hydrogen could be produced from renewable resources, such as biomass. However, current technologies for generating hydrogen from biomass, such as enzymatic decomposition of sugars, steam reforming of bio-oils, and gasification of biomass, all suffer from low hydrogen production rates and poor economics.
Recently, Dumesic et al. (References 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10—see References section at the end of this description) have reported that hydrogen can be produced at relatively low temperatures, e.g. around 277° C., over supported metal catalysts by a single step aqueous-phase reforming of biomass-derived oxygenated hydrocarbons, such as methanol, ethylene glycol, glycerol, sorbitol, xylose and glucose. In addition to utilizing renewable feedstocks, aqueous phase reforming of oxygenated hydrocarbons eliminates the need to vaporize water and the oxygenated hydrocarbon (which reduces the energy requirements and CO2 generation for producing hydrogen). Generating 1 ton of hydrogen (11,120 m3 at STP) from glycerol (derived from biomass) displaces 5.5 tons of CO2 from fossil origin (when employing steam methane reforming) (see References 11 and 12). In addition, the low reaction temperature and the absence of water vaporization eliminate 0.6 ton of CO2/ton H2 produced (References 11 and 12). The production of H2 and CO2 by aqueous phase reforming also leads to the production of low levels of CO (<1000 ppm) in a single catalytic process.
Nevertheless, important selectivity challenges govern hydrogen production by aqueous phase reforming because the mixture of H2 and CO2 formed in this process is thermodynamically unstable at low temperatures with respect to formation of methane (Reference 1). Accordingly, the selective formation of hydrogen represents a classic problem in heterogeneous catalysis and reaction engineering: namely the identification of a catalyst and the design of equipment and conditions to maximize the yields of desired products at the expense of undesired byproducts formed in series and/or in parallel reaction pathways. Several types of catalysts, including supported metal and Sn-modified Raney Ni catalysts, have been tested for aqueous phase reforming in order to identify the effect of different catalytically active metals, metal alloy components and supports on H2 selectivity (Reference 1). Among them, the Pt/γ-Al2O3 and Sn-modified Raney Ni catalysts were the most promising (References 1, 2, 3, 5, 6, 7, 8, 9 and 10). The 3 wt % Pt/γ-Al2O3 catalysts were found to have the best hydrogen selectivity, yield and production rate (References 11 and 12).
However, little work has been done on reactor configuration to maximize hydrogen selectivity, yield and production rate. In the current aqueous phase reforming development, only fixed-bed tubular reactors have been used for activity testing (References 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12). Nevertheless, aqueous phase reforming involves multiple phases: reactants in the liquid phase, catalysts in the solid phase, and the desired hydrogen product in the gas phase. Therefore, the interphase and intraparticle mass transfer is one of the critical issues that need to be addressed for measuring and optimizing catalyst activity. To measure the catalyst intrinsic activity, very fine particles between 63-125 μm were tested in fixed bed tubular reactors by Dumesic et al. (References 3 and 6). For example, for the kinetic study of aqueous phase reforming of ethylene glycol over an alumina-supported platinum catalyst, a maximum particle size of 130 μm was used to ensure that the intraparticle and interphase mass transfers were not limiting (Reference 6).
In theory, such fine particles are not recommended in fixed bed tubular reactors because of the resulting high pressure drop along the catalyst bed in reaction conditions. Also it is easy for small particles to be washed out by the liquid stream, which implies potential catalyst loss and undesirable secondary pollution to the liquid (Reference 13). Moreover, the use of fixed bed tubular reactors for liquid solid phase reaction has some other disadvantages, such as poor wetting of catalyst, although co-current upflow has been used to improve the wetting of catalyst (Reference 14).
Various patents and patent applications have been published in this field. Most notable are the patents and patent applications of Cortright and Dumesic, assigned to the University of Wisconsin-Madison, e.g. U.S. Pat. No. 6,699,457 issued on Mar. 2, 2004, U.S. Pat. No. 6,964,757 issued on Nov. 15, 2005, and U.S. Pat. No. 6,964,758 issued on Nov. 15, 2005, as well as published applications 2003/0220531 and 2005/0207971. These patents and applications describe the use of fixed bed tubular reactors (plug flow reactors) as exemplary systems. As noted above, such reactors are not ideal for hydrogen generation involving aqueous phase reforming.
A different approach has been taken by Patrick Grimes et al. in Canadian patent 787,831 of Jun. 18, 1968 and Canadian patent application Serial No. 2,613,497 filed Jun. 23, 2006. In these publications, an organic compound is reacted with water in a closed reactor in the presence of an electrolyte (preferably an alkaline compound such as KOH) and an electronically conductive catalyst. An electrical potential is applied between electrodes (or via an electrode) and gaseous hydrogen is produced (with carbon dioxide reacting to form a carbonate that remains in solution). It was stated that the conducting electrode may be replaced by suspending particles of any electronically conducting material in the liquid reactants, but the presence of the electronically conductive material is absolutely necessary because, without it, the reaction will not proceed or will proceed so slowly that it could not be of any possible commercial interest even at elevated temperatures. Thus, these procedures adopt an electro-chemical approach and require the presence of an electrolyte and a suspended electronically conductive material.
There is a need for improved processes and equipment for the aqueous phase reforming reaction used for the production of hydrogen from oxygenated hydrocarbons, particularly those obtained from biomass.